Auxiliary power units (APUs) are gas turbine engines used on aircraft to supply pneumatic power and/or shaft horsepower in lieu of the primary propulsion engines. Typically, the APU is mounted in the tailcone of the aircraft, see FIG. 1, though on some airplanes such as the Boeing 727, the APU is mounted in the wheelwell. Because the APU is inside the aircraft an inlet duct 15 is required to bring air from outside the airplane to the APU. These inlet ducts generally have a door that is open when the APU is running and closed when the APU is shutdown. When the APU is needed during flight the door is opened and because the aircraft is moving, air is rammed or pushed through the inlet to the APU. This ram effect causes the pressure to rise in the duct which creates a positive pressure ratio between the compressor inlet of the APU and the. exhaust of the APU. This pressure ratio is referred to as engine delta P. If the delta P becomes too large; the APU may experience difficulty in starting.
One proposed solution has been the use of an open loop control system. Under this system, a predetermined door position table is incorporated into the APU's electronic control unit. The electronic control unit receives signals indicative of the APU's operating condition enters the door position table and generates a predetermined door position signal. The door then moves in response to this signal. A disadvantage to the open loop system is that the table is only accurate for steady state aircraft conditions. Any changes in pitch or attitude are not compensated for and can result in unacceptable engine delta P and difficulty in starting the APU can occur.
Accordingly, a need exists for an improved inlet control system that overcomes the disadvantages of the prior art.